When Did Human Ancestors Bid Farewell to the Trees? Limb Strength Ratios Shed Light on Evolution

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Scientists analyzed the arm and leg strength ratios of fossilized bones to determine when early human ancestors lost their tree-climbing abilities.

When Did Human Ancestors Bid Farewell to the Trees? Limb Strength Ratios Shed Light on Evolution
Image credit: Archaeology

The transition from trees to the ground, one of the greatest mysteries of human evolution, is being placed into a clearer framework through a new biomechanical analysis. By examining the strength ratios of the arm and leg bones of early human ancestors, researchers have sought to determine how long these beings continued to spend time in trees and when they transitioned to a fully terrestrial lifestyle [1].

In the analyses conducted, data obtained from fossil remains were compared with modern primate species. The bone structure of species that navigate through trees in search of fruit, such as orangutans, shows distinct differences when compared to the load distribution necessitated by terrestrial life [2]. Such comparisons help us understand not only the skeletal structure of hominins but also their daily locomotor habits.

The process of human evolution was not one where tree-climbing ability was abandoned overnight. Species such as Ardipithecus ramidus, nicknamed 'Ardi,' who lived approximately 4 million years ago, were capable of walking on two legs while also possessing grasping toes that allowed them to climb trees [6]. However, over time, by approximately 2.5 million years ago, with the reduction in the size of arms and fingers, our ancestors fully adapted to life on the ground and lost their agility in the trees.

This evolutionary shift also symbolizes the divergent paths taken by humans and other great apes after diverging from a common ancestor [3]. Despite our genetic and anatomical similarities with close relatives like gorillas and chimpanzees, the fact that our legs became stronger relative to our arms is a result of our species' strategy for traversing long distances in open savannas. According to researchers, this anatomical transformation formed the foundation of the mobility that allowed modern humans to dominate the world.

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Image & News Source: Archaeology / archaeology.org